test: add validation tests for POST /echo

- Test successful echo responses for valid messages
- Verify empty messages are rejected with HTTP 400
- Validate request bodies using a Zod schema with a 1–500 character limit
- Return a consistent error response for invalid request bodies

src/server.test.ts

@@ -37,6 +37,36 @@ describe("API endpoints", () => {
     });
   });
 
+  it("POST /echo returns the validated message", async () => {
+    const response = await app.inject({
+      method: "POST",
+      url: "/echo",
+      payload: {
+        message: "Hello, world!",
+      },
+    });
+
+    assert.equal(response.statusCode, 200);
+    assert.deepEqual(response.json(), {
+      message: "Hello, world!",
+    });
+  });
+
+  it("POST /echo rejects an empty message", async () => {
+    const response = await app.inject({
+      method: "POST",
+      url: "/echo",
+      payload: {
+        message: "",
+      },
+    });
+
+    assert.equal(response.statusCode, 400);
+    assert.deepEqual(response.json(), {
+      error: "Invalid request body",
+    });
+  });
+
   it("returns 404 for an unknown endpoint", async () => {
     const response = await app.inject({
       method: "GET",

src/server.ts

@@ -1,4 +1,5 @@
 import Fastify from "fastify";
+import { z } from "zod";
 
 export const buildApp = () => {
   const app = Fastify({
@@ -16,6 +17,24 @@ export const buildApp = () => {
     },
   }));
 
+  app.post("/echo", async (request, reply) => {
+    const schema = z.object({
+      message: z.string().min(1).max(500),
+    });
+
+    const result = schema.safeParse(request.body);
+
+    if (!result.success) {
+      return reply.code(400).send({
+        error: "Invalid request body",
+      });
+    }
+
+    return {
+      message: result.data.message,
+    };
+  });
+
   return app;
 };